Multilayer tubular structures presently being manufactured, with few exceptions, have a symmetrical structure consisting of a middle barrier layer, adhesive layers, adjacent each side of the barrier layer, and outside carrier layers. Optionally, an additional regenerated material layer is introduced on one or both sides of the barrier layer, between the adhesive and carrier layers, or else the tubular structures produced consist of either the inside or outside part of the above described symmetrical structure, i.e., one having the barrier layer as the outside or the inside layer with the remaining layers consisting of an adhesive and carrier layer to one side or the other of the barrier layer. As the materials are first formed into individual tubular structures, each having a wall thickness that is maintained precisely to a fraction of a millimeter, it has been customary to use a separate coextrusion head, with associated extruder mechanism, for each type tubular structure being manufactured. This has been necessary because, with the different tubular structures, the carrier layer(s), adhesive layer(s), barrier layer and, when used, the regenerated material layer(s) are found at different positions within the tubular structure. This variation produces differing manufacturing considerations with respect to material characteristics thereby requiring different extruder apparatuses and feed passage configurations.
This requirement for a variety of extrusion heads and associated apparatuses raises the cost considerably, so that when small quantities are to be manufactured, coextrusion technology becomes uneconomical and, as a result, other conventional container products are preferred.